A mythical period in time which everyone knows about but has no memory of.
A lot of people say that it is the day after today and that tomorrow will be here at midnight, but if you actually wait until midnight it is today again.
Often used by people to put off work as the new required deadline will never arrive.

Dad: Son, tidy your room.
Son: Ill do it tomorrow.
Dad: Get out of my house you lazy shit!